The present invention relates to a band pass filter, and particularly, to a highly compact band pass filter that reliability achieves desired characteristics.
In recent years, marked advances in miniaturization of communication terminals, typically mobile phones, has been achieved thanks to miniaturization of the various components incorporated therein. One of the most important components incorporated in a communication terminal is a filter component. As a filter component, a band pass filter described in xe2x80x9cLow-Profile Dual-Mode BPF Using Square Dielectric Disk Resonator (Proceeding of the 1997 Chugoku-region Autumn Joint Conference of 5 Institutes, p272, 1997)xe2x80x9d is known.
The band pass filter described in this paper is constituted of a TEM dual-mode dielectric disk resonator. This dielectric disk resonator measures 5 mmxc3x975 mm in plan view and its upper and lower surfaces are coated with silver plates. The silver plate on the upper surface is electrically floated whereas the silver plate on the lower surface is grounded. A dielectric material whose dielectric constant ∈r=93 is interposed between these two silver plates. All of the side surfaces of the dielectric resonator are open to the air. Thus, electric field is maximum (+ve or xe2x88x92ve) throughout the wall of the resonator. The electric field should be minimum at the symmetry plane of the resonator. For this reason, this type of dielectric resonator is call a half-wave (xcex/2) dielectric disk resonator.
FIG. 1 is a graph showing a theoretical characteristic curve of the relationship between the thickness of the dielectric resonator described in the paper and unloaded quality factor (Q0), together with experimentally obtained values.
As shown in FIG. 1, the unloaded quality factor (Q0) of the dielectric resonator is maximum (≈250 (experimental value)) when the thickness thereof is 1 mm. Thus in this type of the dielectric resonator, the unloaded quality factor (Q0), a parameter indicating performance, depends on the thickness of the dielectric resonator.
In contrast, the resonant frequency of the dielectric resonator depends on the size of its plan view. For example, if the dielectric resonator set out in the above-mentioned paper is fabricated to have a resonant frequency of 2 GHz, the dimension of the resonator become 8.5 mmxc3x978.5 mmxc3x971 mm. Thus, a band pass filter formed using such a dielectric resonator is large.
As a need continues to be felt for still further miniaturization of communication terminals such as mobile phones, further miniaturization of filter components, e.g., band pass filters, incorporated therein is also required.
However, it is extremely difficult to miniaturize filter components while still achieving the required characteristics because, as explained in the foregoing, the characteristics thereof (such as unloaded quality factor (Q0) and resonant frequency) depend on filter size.
It is therefore an object of the present invention to provide a compact band pass filter having desired characteristics.
The above and other objects of the present invention can be accomplished by a band pass filter comprising: a first resonator of disk-shape having an input terminal formed on one side surface thereof, a second resonator of disk-shape having an output terminal formed on one side surface thereof, an evanescent waveguide interposed between the first and the second resonators, and a capacitive stub having a first portion formed on another side surface of the first resonator and a second portion formed on another side surface of the second resonator.
According to this aspect of the present invention, because the resonant frequencies of the first and the second resonators are lowered by the capacitive stub, the overall size of the band pass filter can be reduced compared with the size that would otherwise be determined by the resonant frequencies of the first and second resonators. Moreover, because the coupling constant between the first and second resonators is lowered by the capacitive stub, the thickness of the evanescent waveguide can be thickened compared with that it would otherwise have, so that the mechanical strength of the band pass filter is enhanced. Further, the capacitive stub reduces the effect of unnecessary higher mode resonation of the band pass filter.
In a preferred aspect of the present invention, the band pass filter further comprises a metal plate formed on a side surface of the evanescent waveguide, thereby connecting a first portion of the capacitive stub and a second portion of the capacitive stub.
In a further preferred aspect of the present invention, the first portion of the capacitive stub and the second portion of the capacitive stub have the same dimensions.
In a further preferred aspect of the present invention, the capacitive stub further has a third portion formed on the one side surface of the first resonator and a fourth portion formed on the one side surface of the second resonator.
According to this preferred aspect of the present invention, the overall size of the band pass filter can be further reduced and the mechanical strength of the band pass filter can be further enhanced.
The above and other objects of the present invention can be also accomplished by a band pass filter comprising:
first and second dielectric blocks each of which has a top surface, a bottom surface, first and second side surfaces opposite to each other, and third and fourth side surfaces opposite to each other;
a third dielectric block in contact with the first side surface of the first dielectric block and the first side surface of the second dielectric block;
metal plates formed on the top surfaces, the bottom surfaces, and the second side surfaces of the first and second dielectric blocks;
a first electrode formed on the third side surface of the first dielectric block;
a second electrode formed on the third side surface of the second dielectric block;
a first capacitive stub formed on the fourth side surface of the first dielectric block; and
a second capacitive stub formed on the fourth side surface of the second dielectric block.
According to this aspect of the present invention, because the resonant frequencies of the two resonators constituted by the first and second dielectric blocks are reduced by the first and the second capacitive stubs, the overall size of the band pass filter can be reduced compared with the size that would otherwise be determined by the resonant frequencies of the resonators. Moreover, because the coupling constant between the resonators is lowered by the first and second capacitive stubs, the thickness of the evanescent waveguide constituted by the third dielectric block can be thickened compared with the thickness it would otherwise have, so that the mechanical strength of the band pass filter is enhanced. Further, the radiation loss arising at the fourth side surfaces of the first dielectric block and the fourth side surface of the second dielectric block is reduced by the first and the second capacitive stubs. Furthermore, the first and second capacitive stubs reduces the effect of the unnecessary higher mode resonation of the band pass filter.
In a preferred aspect of the present invention, the first dielectric block and the second dielectric block have the same dimensions.
In a further preferred aspect of the present invention, the first capacitive stub is in contact with the metal plate formed on the bottom surface of the first dielectric block, and the second capacitive stub is in contact with the metal plate formed on the bottom surface of the second dielectric block.
In a further preferred aspect of the present invention, the first capacitive stub and the second capacitive stub have the same dimensions.
In a further preferred aspect of the present invention, the third dielectric block has a first side surface in contact with the first side surface of the first dielectric block, a second side surface in contact with the first side surface of the second dielectric block, a third side surface parallel to the third side surface of the first dielectric block, a fourth side surface parallel to the fourth side surface of the first dielectric block, a top surface parallel to the top surface of the first dielectric block, and a bottom surface parallel to the bottom surface of the first dielectric block on which a metal plate is formed.
In a further preferred aspect of the present invention, the bottom surfaces of the first to third dielectric blocks are coplanar.
In a further preferred aspect of the present invention, the third side surface of the first dielectric block and the third side surface of the third dielectric block are coplanar, and the fourth side surface of the first dielectric block and the fourth side surface of the third dielectric block are coplanar.
In a further preferred aspect of the present invention, the third side surface of the first dielectric block and the third side surface of the second dielectric block are coplanar, and the fourth side surface of the first dielectric block and the fourth side surface of the second dielectric block are coplanar.
In a further preferred aspect of the present invention, a metal plate is formed on the fourth side surface of the third dielectric block thereby integrating the first capacitive stub, the second capacitive stub, and the metal plate formed on the fourth side surface of the third dielectric block.
In a further preferred aspect of the present invention, the third side surface of the first dielectric block and the fourth side surface of the second dielectric block are coplanar, and the fourth side surface of the first dielectric block and the third side surface of the second dielectric block are coplanar.
In a further preferred aspect of the present invention, the band pass filter further comprises a third capacitive stub formed on the fourth side surface of the first dielectric block and a fourth capacitive stub formed on the fourth side surface of the second dielectric block.
According to this preferred aspect of the present invention, the overall size of the band pass filter can be further reduced and the mechanical strength of the band pass filter can be further enhanced.
In a further preferred aspect of the present invention, the first electrode is in contact with the metal plate formed on the top surface of the first dielectric block, and the second electrode is in contact with the metal plate formed on the top surface of the second dielectric block.
In a further preferred aspect of the present invention, the first dielectric block and the metal plates formed on the top surface, bottom surface and second side surface thereof constitute a quarter-wave (xcex/4) dielectric resonator, and the second dielectric block and the metal plates formed on the top surface, bottom surface and second side surface thereof constitute another quarter-wave (xcex/4) dielectric resonator.
In a further preferred aspect of the present invention, an end of the first capacitive stub is positioned at a center of the fourth side surface of the first dielectric block, and an end of the second capacitive stub is positioned at a center of the fourth side surface of the second dielectric block.
According to this preferred aspect of the present invention, because the first and second capacitive stubs are formed at regions of the fourth side surfaces of the first and second dielectric blocks where the electric field is relatively strong, marked effects of lowering resonant frequency, thickening the evanescent waveguide constituted by the third dielectric block, reducing radiation loss, and reducing the effect of the unnecessary higher mode resonation are obtained.
The above and other objects of the present invention can be also accomplished by a band pass filter comprising:
first and second dielectric blocks each of which has a top surface, a bottom surface, first and second side surfaces opposite to each other, and third side surface perpendicular to the first side surface;
a third dielectric block in contact with the first side surface of the first dielectric block and the first side surface of the second dielectric block;
metal plates formed on the top surfaces, bottom surfaces, and second side surfaces of the first and second dielectric blocks;
a first electrode formed on the third side surface of the first dielectric block; and
a second electrode formed on the third side surface of the second dielectric block,
a coupling capacitance being established between a first resonation circuit formed between the first electrode and the metal plates and a second resonation circuit formed between the second electrode and the metal plates, the band pass filter further comprising:
means for providing an additional capacitance in parallel with the first resonation circuit and another additional capacitance in parallel with the second resonation circuit.